A polyurethane gel is created from a polyurethane network and a solvent. The polyurethane network envelops the solvent and can prevent the solvent from flowing out of the network. The properties of a polyurethane gel depend largely on the structure of the polyurethane network that makes up the gel and the interaction of the network and the solvent. The polyurethane network depends on the crosslink structure of the network, which depends on, for example, the amount and type of the reactants used to make the network and their ability to react to near completion. The polyurethane network can be important for determining the strength of the gel and can also be important for the diffusion of molecules through the tortuous path in the gel.
For some applications, one desirable characteristic of a polyurethane gel is transparency. Transparency is determined by the polyurethane network in combination with the solvent as well as the residual reactants. Thus, some reactants that may provide a desirable polyurethane network may be unable to provide transparency, and some reactants that can provide transparency may be unable to provide a desirable network.
A variety of polyurethane gels are known, and some of these gels are transparent. The known transparent polyurethane gels are less than desirable because they generally require large amounts of polymer—e.g., more than 5 weight percent and even more than 20 weight percent in some applications. Using such large amounts of polymer can be expensive and can negate or reduce transparency. These gels are also typically prepared in an organic solvent, which can be at least partly removed in a washing step after the gel is formed. This washing step can be slow and expensive in high-volume manufacturing applications.
Attempts to reduce the amount of polymer in known formulations to no more than 5 weight percent can adversely affect gel formation. And attempts to modify known formulations by altering the reactants such that less than 5 weight percent of polymer can form a desirable gel can adversely affect gel transparency.
Thus, it would be desirable to prepare a polyurethane gel by using a reduced amount of polymer as compared to known formulations while still obtaining a transparent gel and desirable physical properties. It would also be desirable to prepare a polyurethane gel in the substantial absence of volatile organic solvent, while still maintaining gel transparency.